Adding content to sidebars

I am working on a three-column layout where all three columns will have differing content depending on the post/page in the main column.

How would I go about giving the ability to change and update the sidebar content and display it in templates using something like <?php the_content() ?> ?

Since the sidebar content is related to the post I will need a way to reference the right sidebar content for the post.

Thanks in advance

Yours sincerely,
nav

Add a meta box to the post editor, similar to the excerpt meta box, and get the content of the meta box in your sidebar.

If you want to reuse the same text on different pages create a custom post type sidenotes and add a meta box with a select field to let the author choose the text.